While Purdy certainly does benefit from a good roster, he earned that starting job and has done better under Shanahan than anybody since maybe Ryan. The kid's processing speed is elite which is something really hard to evaluate coming out of CFB and probably the most important trait for an NFL QB.
Williams just wants his money guaranteed, not a lot more one than he was going to count for. Aiyuk will get his raise but the way the cap numbers work he won't eat up a ton of cap space for a while. Purdy will get a big contract but again, won't kill them for a few years. Deebo might be a casualty next year, but hopefully one of the rooks(Pearsall looks good if he can stay healthy) can step up there.
They've got a few more years in this window, especially if they keep their picks and draft decently. Year 3 of Purdy's new deal is when it might get ugly...